1.0 Field of the Invention
The present invention relates to a system and method for assisting an aircraft pilot during landing maneuvers and has particular use for landing helicopters aboard marine structures. More particularly, the present invention relates to visual landing aid systems for guidance of aircraft during final approach and specifically to passive, electro-optical display systems for enhancing visual cueing information projected to the pilot of an approaching aircraft from the point of intended landing on an helicopter pad using a Stabilized Glide Slope Indicator (SGSI) system.
2.0 Description of the Prior Art
Successful aircraft landings, particularly night time operations into short runways and into airports with long, over water approach patterns extending into a surrounding body of water, necessitate the highest degree of safety and control to ensure maximum protection for passengers, cargo and aircrew.
Current Stabilized Glide Slope Indicator (SGSI) devices, are optical landing aid devices that provide glide slope information to helicopter pilots at night and in foul weather. Such systems have been in use for over 30 years. The SGSI devices provide a pilot with visual cues for night landings so that a pilot can maintain desired altitude and rate of decent. Usually a copilot is mandatory to determine clearance altitude during such night landings. The U.S. Navy has developed and refined through land based flight testing landings using an advantageous 40 degree horizontal coverage so as to not fly out of coverage at one-half (1/2) nautical mile (nm).
Landing helicopters at night on small ships is a demanding and dangerous task. To reduce work load and improve safety visual cues were developed that illuminated the landing area and provided an approach light that would display glide slope or vertical height information to an approaching pilot. Initially gravity stabilization was used, as in the British Glide Path Indicator (GPI), but the stabilization accuracy of this device was insufficient to provide consistent glide slope information. A number of navies, including the U.S. Navy, developed stabilized platforms on which to place landing lights. These platforms were physically large, costly and created maintenance problems. They also did not compensate for heave motions. The moving parts of these devices are also disadvantageously exposed to environmental effects such as wind loading, ice loading and the hostile electromagnetic environments present on modern warships. Their displays also have to be disadvantageously manually rotated to point at different approach paths. Furthermore, with the advent of Night Vision Devices (NVD's), the displays were rendered useless since the display information was color coded and the NVD's can not discern colors. The large size and complex shape of the display systems gave them a large radar gross-section, which is undesirable in new ship design. The large size also makes it difficult to locate these indicator systems on crowded hanger decks or ship superstructures. This is particularly difficult if a low radar cross section is desired for such a system. Many of the world navies use displays different from the U.S. Navy and the design of these systems is such that the display cannot be changed without a complete redesign of the optical system.
U.S. Patents that relate to apparatus and methods for aiding in helicopter landings include U.S. Pat. No. 5,982,299, entitled "Laser Based Visual Landing Aids and Method for Implementing Same." This teaching discusses some of the functional SGSI system requirements. In particular, helicopter operations in land and sea based environments using laser visual landing aid corridor designs for azimuth guidance upon approaching a helicopter landing zone (or pad), have a central amber (first) corridor that is slightly wider than that used for carrier or land-based fixed wing aircraft facilities. For helicopter landing zone landing aid systems, this teaching discusses the use of a central corridor that is 0.6 degree in width, plus or minus 0.1 degree. The steady red and green (second and third) corridors use a 0.8 degree plus or minus 0.1 degree, and the slowly flashing red and green (fourth and fifth) corridors are widened to 0.4 degree, plus or minus 0.04 degree. There are no rapidly flashing corridors in that design. Vertical height of each of the corridors is in the range of 4 to 5 degrees. However, this teaching does not discuss how to make an SGSI system with using only electromechanical moving optical components within a housing mounted to a moving marine structure. Furthermore, laser sources are required for this type of implementation for U.S. Pat. No. 5,982,229 and this presents a potential laser eye safety hazard.
U.S. Pat. No. 4,667,196 ('196) entitled "Active Visual Display System for Remote Three-Axis Flight Path Guidance of Landing Aircraft," teaches an active, electro-optical display system for use on fixed-wing, land based airport runways, which provides for remotely guiding a pilot during visual approach and landing of an aircraft using one embodiment termed "Conventional Microwave Landing System (MLS)." The system of the '196 patent uses ground transmitted data that is air-derived on board the aircraft and data linked to a ground receiver to produce a continuous digital data signal indicative of aircraft slant range, elevation and azimuth relative to the desired landing position. As described in the '196 patent, the resulting data signal is electrically coupled to a signal processor controlled in accordance with control guidance parameters to produce three discrete control signals indicative of (1) the magnitude and direction of the descent rate error, (2) the flight path acceleration, and (3) the lateral drift rate of the aircraft relative to the intended landing area. The three control signals are respectively coupled to display drivers which produce a plurality of drive signals for energizing individual light cells in horizontally oriented linear arrays located adjacent to the runway. The resulting light signals provide a continuous visual indication of the flight path acceleration and flight path angular error, in the elevation and azimuth planes, for appropriate corrective action by the pilot. However, this teaching does not teach or suggest an apparatus or method that compensates for marine platform motions of pitch and roll during aircraft landing so as to provide a pilot with helpful guidance information on a moving platform. This teaching of the '196 patent, requires active aircraft tracking with MLS and Distance Measuring Equipment (DME) or other tracking devices. It also requires a VASI system, which is a physically large display.
U.S. Pat. No. 5,922,039 ('039) entitled "Actively Stabilized Platform System," teaches of a method to perform two axis stabilization without the use of a gyroscope by using accelerometers, position sensors and proportional drive controller augmented with estimation and filtering. However, the '039 patent does not teach or suggest an apparatus or method that compensates for marine platform motions of pitch and roll by providing moving optical elements during aircraft landing so as to provide a pilot with helpful guidance information on a moving platform. Whereas the '039 patent discloses a method of deriving signals to drive a stable platform, it does not teach or suggest of either a method for moving optical elements or creating a stable display in space for various azimuth angles.
Other limitations of presently used SGSI systems aboard marine structures include hydraulic systems for actuation and control that generally require the use of several gallons of hydraulic oil for operation. These ancillary hydraulic systems require frequent and extensive maintenance where hydraulic fluid is changed every six months and exposed mechanical joints must be greased which results in storage of many solvents and waste materials onboard a marine structure such as a ship where storage is often at a premium. These shipboard wastes require the storing of containers, rags, gloves, foot covering, face shields, and aprons needed for regular maintenance. The ship must store oil for waste disposal at appropriate shore facilities. The hydraulic oils used in this system usually disadvantageously react with aluminum and steel parts causing corrosion.
Thus, replacement of current SGSI systems having hydraulic systems is desirable in view of extensive maintenance requirements and their high initial capital expense, several times as expensive as the proposed invention. An improved system having the required display presentation and performance requirements of currently used systems comprising electronic and optical components is desirable and which the present invention provides.